From 7e630e3e68064660ace3653fd34f55066f5d38c2 Mon Sep 17 00:00:00 2001
From: Pedro Portela <pedro.portela@cs.tu-dortmund.de>
Date: Thu, 5 Jun 2025 20:40:23 +0200
Subject: [PATCH] Reads chunk length

---
 img_png.c | 66 +++++++++++++++++++++++++++++++++++++++++++++----------
 img_png.h |  7 +++---
 2 files changed, 58 insertions(+), 15 deletions(-)

diff --git a/img_png.c b/img_png.c
index a0ee188..5c50c11 100644
--- a/img_png.c
+++ b/img_png.c
@@ -1,23 +1,67 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include "img_png.h"
 
-uint8_t* img_png_decode(FILE *fp) {
-  unsigned char buffer[8];
-  size_t bufsize = sizeof(buffer) / sizeof((buffer)[0]);
-  
-  size_t ret = fread(buffer, sizeof(*buffer), bufsize, fp);
+#define BUFSIZE 1024
 
-  if(ret != bufsize) {
-    fprintf(stderr, "fread() failed. Expected %zu bytes, got %zu\n", bufsize, ret);
-    return NULL;
+const unsigned char img_png_signature[] = {0x89, 'P', 'N', 'G', '\r', '\n', 0x1a, '\n'};
+
+uint32_t char_to_uint32 (unsigned char* input) {
+  uint32_t val = (input[0] << 24) + (input[1] << 16) + (input[2] << 8) + input[3];
+
+  return val;
+}
+
+uint8_t* img_png_decode(FILE *fp) {
+  unsigned char buffer[BUFSIZE];
+  unsigned char magic[8];
+  enum chunk_state {
+    LENGTH,
+    TYPE,
+    DATA,
+    CRC
+  };
+  
+  // Check PNG Magic
+  if(fread(magic, sizeof(*magic), 8, fp) < 8) {
+    fprintf(stderr, "Couldn\'t read PNG magic.");
+    exit(EXIT_FAILURE);
   }
 
-  printf("PNG Magic: %#04x%02x%02x%02x%02x%02x%02x%02x\n", buffer[0], buffer[1], buffer[2],
-         buffer[3], buffer[4], buffer[5], buffer[6], buffer[7]);
+  if(memcmp(magic, img_png_signature, 8) != 0) {
+    fprintf(stderr, "Invalid PNG magic\n");
+    exit(EXIT_FAILURE);
+  }
+  
+  printf("PNG Magic: %#04x%02x%02x%02x%02x%02x%02x%02x\n", magic[0], magic[1], magic[2],
+         magic[3], magic[4], magic[5], magic[6], magic[7]);
+
+  // After the magic number is read, we should hit the length of the chunk first.
+  enum chunk_state state = LENGTH;
+  struct img_png_chunk cur_chunk;  
+  while(fread(buffer, sizeof(*buffer), BUFSIZE, fp) > 0){
+    // Refine later!!!
+    
+    switch (state) {
+    case LENGTH: {
+      unsigned char length[4];
+      for (size_t i = 0; i < 4; i++) {
+        length[i] = buffer[i];
+      }
+      cur_chunk.length = char_to_uint32(length);
+      printf("Chunk length: %lu\n", (unsigned long)cur_chunk.length);
+    }
+    }
+  }
+
+  if(!feof(fp)) {
+    perror("fread() failed!");
+    exit(EXIT_FAILURE);
+  }
+  
 
   return NULL;
 }
 
-
diff --git a/img_png.h b/img_png.h
index 506ea91..e809744 100644
--- a/img_png.h
+++ b/img_png.h
@@ -1,13 +1,12 @@
 #include <stdint.h>
 #include <stdio.h>
 
-//const unsigned char img_png_signature[] = {0x89, 'P', 'N', 'G', '\r', '\n', 0x1a, '\n'};
 
 struct img_png_chunk {
-  int length;
-  int chunk_type;
+  uint32_t length;
+  char chunk_type[4];
   char* chunk_data;
-  int crc;
+  uint32_t crc;
 };
 
 uint8_t* img_png_decode (FILE* fp);